Ytterbium laser device

ABSTRACT

Yb3 doped laser device which emits radiation at wavelengths shorter than 1.06 microns at high temperatures. The device includes a laser-active component doped with ytterbium ions and a sensitizer component proximately disposed in relationship to the laser-active component doped with neodymium ions.

United States Patent [72] Inventors Elias Snitzer 3,487,330 12/1969Gudmundsen 331/945 Wesley, Mass; FOREIGN PATENTS :32? wmmk 673,40112/1965 Belgium 331/945 1 APPL NO 825 7 5 OTHER REFERENCES [22] FiledMay 19, 1969 Snitzer et al., Saturable Absorbtion of Color Centers in[45] Patented Oct. 5, 1971 Nd 3+Nd 3+Yb Laser Glass, lEEEJ. QuoatumElec' [73] Assignee American Optical Corporation tronics, 013-2, (9),Sept. 1966 pp. 627- 32 Southbridge, Mass. Vuylsteke et al., Glass LaserTechnology," Laser Focus,

December 1967, pp. 21- 29.

Snitzer, Laser Emission at 1.06u from Nd3 -Yb3* Glass. IEEEJ. of QuontumElectronics QE-Z Sept, 1966, pp. 562- 6. Bonch-Bruevich et al.,Rectangular Rod Neodymium [541 LASER DEVICE 01858 Laser," SovietPhysics-Tech. Phys. 11, (7) Jan 1967,

6 Claims, 6 Drawing Figs. pp 4 [52] [1.8. CI Primary Examiner konald L.wiben [51] In "0151/05 Assistant Examiner-R. .1. Webster Hols 1/02 H0 ls3/16 Anomey-Lane, Altken, Dunner & Z1ems [50] Field otSearch 331/945;

252/3014 ABSTRACT: Yb doped laser device which emits radiation atwavelengths shorter than 1.06 microns at high tempera- [56] ReferencesCmd tures. The device includes a laser-active component doped UNITEDSTATES PATENTS with ytterbium ions and a scnsitizer componentproximately 3,270,291 8/1966 Kosonocky 331/945 disposed in relationshipto the laser-active component doped 3,391,281 7/1968 Eerkens 331/945witlneodymiumions.

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I 11! 1/? 7 2 2 257, 4 I I 28 l I PATENTEDUBT Si n 3.611.188

SHEET 1 BF 2 ZWT %Yb203 IN B'LICATE BLASS 8 M M THICK PERCENT mmsmssaon9 --Nd .FLOURESCENCE SILICATE GLASS naumve so FLOURESCENT 4o INTENSITY30 2o Yb FLOURESCEN1CE m SILICATE GLASS no uo Nd FLOURESCENCE/ (196 1.0aGLASS WAVE LENGTH (m CRO NS) INVENTORS ELIAS SNITZERB: RICHARD EWOODCOCK PATENTEDUET 5m: 3,611,188

SHEEI 2 [1F 2 mvsmons ELIAS SNITZERB: RICHARD F WOODCOCK wajtm; gum vvain/ma ATTOR N EVS vmnnrum LASER DEVICE BACKGROUND OF THE INVENTION Thefield of this invention is Yb doped laser devices. Doubly doped laserglasses containing ytterbium laser ions with neodymium sensitizers areknown. However, in the known compositions laser emission occurs at 1.06microns at room temperature and shifts to 1.015 microns at 77 K. Thereason for the shift from 1.06 microns to 1.015 microns is that the gaincoefficient at 1.015 microns is larger by approximately a factor ofthree than it is at 1.06 microns. The 1.015 micron emission fromytterbium is partially three level in character due to its locationclose to the ground state which is the band at 0.97 microns. At liquidnitrogen temperatures the higher gain coefficient at 1.015 microns leadsto laser emission at this wavelength. At room temperature the partialthree level character of the 1.015 micron line prevents laser emissionuntil the inversion is sufficiently high so that'the lower gaincoefficient of 1.06 microns is sufficient to cause laser emission at thelonger wavelength. Emission at the longer wavelength is further assistedby some residual population in the Nd ions which give an effective gaincoefficient of both ions at 1.06 microns. This is a value which islarger than that for the ytterbium alone. Thus far the only devicesknown which exhibit 1.015 micron emission from ytterbium doped lasersmust be cooled by liquid nitrogen to a temperature of approximately 77K.

In order to prevent laser emission at 1.06 microns at room temperature,it is desirable to use relatively low concentrations of Yb (2 wt.percent of the oxide or less); however, with low concentrations of Yb,the energy transfer from Nd to Yb becomes relatively less efiicient withthe result that the residual excited population of Nd" together with theinverted population of Yb provides sufficient gain at 1.06 microns togive laser emission at this wavelength. The desirability of a relativelylow concentration of Yb for emission at 1.015 microns and the pumpingefficiency provided by the presence of Nd ions can be advantageouslycombined in accordance with the present invention wherein the laserconfiguration consists of a laser-active component doped with arelatively low concentration of Yb and a proximally disposed sensitizercomponent doped with Nd ions alone or Nd ions and Yb ions.

SUMMARY OF THE INVENTION A ytterbium laser device is provided comprisedof two components, a laser-active component containing ytterbium laserions and a sensitizer component doped with trivalent neodymium ions ortrivalent neodymium ions and trivalent ytterbium ions proximatelydisposed in relationship to the laser active component. The deviceproduces laser emission from the ytterbium ion at wavelengths shorterthan 1.06 microns at room temperature (approximately 22 C.) and attempera tures lower than room temperature.

Accordingly, it is an object of the present invention to provide aytterbium doped laser in which the ytterbium ions will lase atwavelengths shorter than 1.06 microns at room temperature.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an energy level diagram forthe neodymium-ytterbium energy transfer scheme;

FIG. 2 is a plot of the fluorescent and absorption intensity spectra forytterbium, and the fluorescent intensity spectrum for neodymium;

FIG. 3 is a perspective view showing one embodiment of the lasermaterial of this invention;

FIG. 4 is a perspective view partially in section of the laser materialshown in FIG. 3 diagrammatically illustrating the transfer of FIG. 1;

FIG. 5 is a perspective view partially in section showing a secondembodiment of the laser material of this invention; and,

FIG. 6 is a perspective view partially in section showing a thirdembodiment of the laser material of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS microns and 1.06 microns. Inthe prior art the selection of either of these two laser-emissivewavelengths was largely controlled by the temperature of thelaser-active medium.

In order to increase absorption of the flash tube light, prior artmaterials have incorporated small amounts of neodymium in theytterbium-doped host. Since trivalent ytterbium does not absorb all ofthe wavelengths emitted by conventional xenon flash tubes, neodymium isincluded to increase absorption and enhance the pumping efficiency.Unlike ytterbium, neodymium has absorption bands in the visible portionof the spectrum and is, therefore, able to absorb the light emitted bythe flash tube in this portion of the spectrum. However, when neodymiumis included as a sensitizer for ytterbium, the laser device emits at1.06 microns at temperatures in the vicinity of room temperature.

FIG. 2 shows the overlap of fluorescence of the neodymium and ytterbiumat 1.06 microns at room temperature. As can be seen in FIG. 2 both Ndand Yb have fluorescent curves which have peaks indicated by referencenumeral 9 which overlaps at 1.06 microns.

Since the Yb operates as a three-level system, the Yb concentrationshould be kept low. However, if the concentration is kept low some Ndions tend to remain in an excited state. Thus with low Yb concentrationsthe transfer of energy from the neodymium to the ytterbium isincomplete. A reason for the emission from Yb at 1.06 microns at roomtemperature is the residual population inversion of Nd ions incombination with the excited ytterbium ions gives a gain coefficientwhich is affective to produce emission at 1.06 microns. This gain islarger than that for the ytterbium ions alone if the ytterbium ionconcentration is low.

FIG. 3 shows an embodiment of the laserable material of the presentinvention. Rod 20 is composed of laser active component 22 which is acore of rod 20 surrounded by cladding 24. In this embodiment, cladding24 serves as a sensitizer component. Homogeneously distributedthroughout core 22 are ytterbium ions. Ytterbium is doped within saidcore in laserable quantities. When used throughout the specification andclaims, the term laserable quantity means a quantity of laser ionssufficient so that when a population inversion is established in thelaser ions, a radiative transition from a selected energy level such asthe F levels to a lower level such as F, is possible. The laser ion ispresent in a concentration so that a sufiicient inversion in populationmay be established between two energy levels so as to provide enoughgain in the laser wavelength of stimulated emission to overcome alllight losses within laserable core 22. Experimentally it has beendetermined that the concentration of ytterbium ions necessary for laseraction is between the range of 10' ions per cubic centimeter and 3X10"ions per cubic centimeter; expressed in a weight percent this would beapproximately between the values of 0.01 30 weight percent of the oxideof Yb,O Cladding 24 which serves as the sensitizer component is dopedwith a quantity of sensitizer ions. The sensitizer ions, that is thetrivalent neodymium ions, are provided in a concentration sufiicient toenable an energy transfer from the sensitizer to the laser ion. Theranges of laser and sensitizer ions are known and do not constitute apart of the invention. It is to be understood that the base glass may beof any material I previously used in the glass laser art and includessilicates, phosphates, borates, borosilicates, arsenic trisulflde,se1enides, teluides, fluorides, oxifluorides, alumino-silicates,germanates and organic glasses. The only controlling requirement for theglass base is that it be essentially transparent at the wavelengths atwhich ytterbium and neodymium absorb and transmit energy. Cladding 24may be composed of the same glass base as core 22; however, theinvention is not intended to be limited to this specific arrangement.

FIG. 5 shows an alternate embodiment of the invention. In thisembodiment, the laser material is comprised of a number of slabs ofglass doped with active ions. In FIG. 5, slab 32 comprises the laseractive component which is doped with ytterbium ions and is surrounded byone or more slabs 34 which slabs comprise sensitizer components, saidsensitizer components being doped with neodymium ions. An advantage ofthis embodiment is the ratio of the size the sensitizer component andthe laser active component can be easily varied. Also the material shownin FIG. 5 is easily constructed, since the slabs need not be fusedtogether.

FIG. 6 shows another embodiment of the invention and in this embodimentthe laser-active component indicated by 36 has formed therein sensitizercomponents in the form of optical fibers 38 longitudinally disposedthroughout said laser active component. An advantage of this embodimentis that energy being transferred from the sensitizer component will bemore efficiently absorbed by the laser active component that is the casewith the embodiment shown in FIG. 3, since the sensitizer components aresurrounded by the laser active component.

In all embodiments of the invention it is essential that the neodymiumions be present only in the sensitizer component and that the laseractive component contain no neodymium ions. The simplest embodiment ofthe invention then resides in some of the energy is transferred in aninward direction and is absorbed by the active laserions within core 22,and some of the energy travels outward and is lost. Some of this lostenergy can be reflected by the silver foil (not shown) used to coupleflash tube light to-the laser rod and can be recaptured. Also exteriorof cladding 24 can be provided with a ground finish, which can scatterlight back into the rod.

The laserable material of the present invention is intended forutilization in laser light generators. The term generator" includes bothoscillators and amplifiers. In an oscillator, rod 20 itself may beprovided with reflective ends 30, 31 on core 22 (see FIG. 3). Oralternatively laser rod 20 may be positioned within an opticallyregaenerative resonant cavity In any embodiment It rs important at theends of the sensitizer elegrinding the ends of the sensitizer component.The term 0" providing a laserable material with a sensitizer componentproximately disposed in relationship to the laser active component andwith the sensitizer component containing all the neodymium ions. Bestresults, however, are obtainable when the sensitizer component alsocontains some trivalent ytterbium ions. When ytterbium ions are presentin the sensitizer component they transfer energy from the sensitizercomponent to the laser-active component. The useful range of ytterbiumoxide in the sensitizer is between the values of approximately 0.1 to 55weight percent.

Operation of the laser material can be understood by reference to FIG.4. Light from a flash tube (not shown), enters rod 20 as is indicated byarrow 26 and excites ions along its path. It is to be understood thatthe light from the flashlamp not only excites ions in the sensitizercomponent, but also passes through such sensitizer components into thelaser-active component and excites the active ions within saidlaseractive component. Neodymium ions within cladding 24 are excited bythe flash tube to the F level and transfer this energy to exciteytterbium ions both within the sensitizer component 24 and laser activecomponent 22 to the F, level as is shown by wavy arrow 28. The ytterbiumions in sensitizer component 24 transfer energy the ytterbium ionswithin the laser-active components. Energy is also transferred directlyfrom the neodymium ions in the sensitizer component to the ytterbiumions within the laser active component.

FIG. 4 shows possible energy patterns of the P level of the neodymiumand the "F level of the ytterbium ions within cladding 24 and core 22.As is shown by the arrows in FIG. 4,

is intended to represent the ratio of wave energy storage to wave energydissipation per unit cycle.

Accordingly, by providing a laser material with a sensitizer componentcontaining neodymium ions and a laser component containing ytterbiumions the device is one which produces laser energy from the ytterbiumions atywavelengths shorter than 1.06 microns at approximately roomtemperature and lower.

We claim:

1. A trivalent ytterbium-doped laser device for emitting opticalradiations at room temperature in a wave band centered at a wavelengthshorter than l.06 microns from the stimulated emission of trivalentytterbium comprising two structurally separate components, the firstcomponent being a laser-active component and the second component beinga sensitizer component, the laser-active component being doped with alaserable quantity of ytterbium ions and the sensitizer component beingdoped with a quantity of neodymium ions sufficient to absorb energy froma pump source and transfer this energy to ytterbium ions, saidstructurally separate components being disposed adjacent to each otherto form said laserable material and to enable transfer of energy fromthe sensitizer component to the laser-active component.

2. The laser device as set forth in claim I wherein said sensitizercomponent has included therein ytterbium ions which ytterbium ionstransfer energy from said sensitizer component to said laser-activecomponent.

3. The laser device as set forth in claim I wherein said sensitizercomponent comprises a cladding enclosing said laseractive component.

4. The laser device as set forth in claim 3 wherein said laseractivecomponent is a core of a laser rod.

5. The laser device as set forth in claim 1 wherein said sensitizerelement comprises a number of optical fibers formed and longitudinallydisposed within said laser-active component.

6. The laser device as set forth in claim I wherein said laseractivecomponent and sensitizer component are glass slabs.

2. The laser device as set forth in claim 1 wherein said sensitizercomponent has included therein ytterbium ions which ytterbium ionstransfer energy from said sensitizer component to said laser-activecomponent.
 3. The laser device as set forth in claim 1 wherein saidsensitizer component comprises a cladding enclosing said laser-activecomponent.
 4. The laser device as set forth in claim 3 wherein saidlaser-active component is a core of a laser rod.
 5. The laser device asset forth in claim 1 wherein said sensitizer element comprises a numberof optical fibers formed and longitudinally disposed within saidlaser-active component.
 6. The laser device as set forth in claim 1wherein said laser-active component and sensitizer component are glassslabs.